JP7406876B2 - Piezoelectric transformers and electronic equipment - Google Patents

Description

The present invention relates to a piezoelectric transformer with high power conversion efficiency. The present invention also relates to an electronic device using the piezoelectric transformer.

A piezoelectric transformer is known as an electronic component that converts voltage in electronic equipment to step up or step down the voltage. The following are known as examples of piezoelectric transformers aimed at increasing output. In other words, a piezoelectric transformer (rod-shaped composite piezoelectric transformer) has been proposed in which a piezoelectric element made of polarized piezoelectric ceramics and multiple electrodes is used as an input/output section, and the input section and output section are stacked and sandwiched between metal cylinders. (Patent Document 1). When electrical energy is applied to the input section as an alternating voltage, the electrical energy is converted into elastic energy by the inverse piezoelectric effect of the piezoelectric ceramics of the input section. This elastic energy is transmitted to the piezoelectric ceramic of the output section via the metal cylinder. Then, due to the positive piezoelectric effect of the piezoelectric ceramics in the output section, it becomes electrical energy again and is taken out from the output section. That is, the configuration is such that a voltage is applied to the input section and the voltage generated at the output section is extracted through mechanical vibration. In addition, in the configuration of such a piezoelectric transformer, the above-mentioned transmission direction of elastic energy corresponds to the 33 directions of the piezoelectric element. The 33 directions of a piezoelectric element are the 3-axis directions when three mutually intersecting spatial axes (for example, the x, y, and z axes) are set as 1, 2, and 3 axes, respectively, and the polarization direction is set as the 3-axis direction. It means the vibration mode of

However, in such a piezoelectric transformer (rod-shaped composite piezoelectric transformer) intended for high output, for example, in the input section, not only vibrations in 33 directions that contribute to elastic energy but also vibrations in 31 directions are generated. This vibration in 31 directions generates heat at the interface between the piezoelectric ceramic and peripheral members such as electrode plates and metal cylinders, causing a vibration transmission loss and causing a problem of lowering power conversion efficiency. Further, in order to increase the degree of integration in electronic devices, it is desired to miniaturize electronic components, but there is a problem in that the heat generated by such a piezoelectric transformer raises the temperature of peripheral components and peripheral members, causing an adverse effect.

Japanese Unexamined Patent Publication No. 51-123592

The present invention has been made to solve the above-mentioned problems, and is intended to improve the interface between piezoelectric ceramics and peripheral members such as electrode plates and metal cylinders in piezoelectric transformers (rod-shaped composite piezoelectric transformers) aimed at increasing output. This is to suppress heat generation in. This provides a piezoelectric transformer with high conversion efficiency of output power to input power.

Further, the present invention provides an electronic device using the piezoelectric transformer.

One piezoelectric transformer of the present invention for solving the above problems is as follows:
A laminate in which a first member, a first piezoelectric element having an electrode and a piezoelectric ceramic , a second piezoelectric element having an electrode and a piezoelectric ceramic , and a second member are laminated in this order; A piezoelectric transformer comprising at least a pressurizing mechanism that tightens the first member and the second member together in the stacking direction,
The piezoelectric ceramic is composed of a perovskite metal oxide containing Ba, Ca, Ti, and Zr, or containing Na and Nb,
The electromechanical coupling coefficient _k31 of the first piezoelectric element and the second piezoelectric element is 23% or more and 26 % or less , and _k33 is 53 % or more and 58% or less ,
The ratio (k ₃₃ /k ₃₁ ) of the electromechanical coupling coefficient k ₃₃ to the electromechanical coupling coefficient k ₃₁ of the first piezoelectric element and the second piezoelectric element is 2.1 or more and 2.3 or less ,
Young's modulus Y ₁₁ at room temperature of the first piezoelectric element and the second piezoelectric element is 116 GPa or more and 123 GPa or less,
The frequency constant N ₃₁ of the first piezoelectric element and the second piezoelectric element is 2210 Hz·m or more and 2610 Hz·m or less,
A Pb component contained in the first piezoelectric element and the second piezoelectric element is less than 1000 ppm.

Further, an electronic device according to the present invention is characterized by using the piezoelectric transformer described above.

According to the present invention, it is possible to provide a piezoelectric transformer that has high power conversion efficiency even at high output. Further, according to the present invention, it is possible to provide an electronic device that uses the piezoelectric transformer and has high power conversion efficiency.

1 is a schematic diagram showing an embodiment of a piezoelectric transformer of the present invention. FIG. 2 is a schematic diagram showing an embodiment of the configuration of a pressurizing mechanism in a piezoelectric transformer of the present invention. It is a schematic diagram showing the displacement amount distribution and stress distribution of stretching vibration (1st, 2nd, 3rd, 4th order) in mechanical resonance in a structure. FIG. 3 is a schematic diagram showing the relationship between the polarization direction and vibration direction of a piezoelectric element. 1 is a schematic diagram showing an embodiment of a piezoelectric transformer of the present invention. FIG. 3 is a schematic diagram showing the relationship between the input/output section, the displacement distribution of stretching vibration, and the stress distribution in an embodiment of the configuration of the piezoelectric transformer of the present invention. FIG. 3 is a diagram showing the relationship between output power and conversion efficiency of output power with respect to input power in an example of the present invention. FIG. 3 is a diagram showing the relationship between k ₃₁ and k ₃₃ , output power, and conversion efficiency of output power to input power in an example of the present invention. 1 is a schematic diagram showing an embodiment of the configuration of a piezoelectric transformer device of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a piezoelectric transformer and an electronic device including the same for carrying out the present invention will be described below.

(Piezoelectric transformer configuration)
FIG. 1 is a schematic diagram showing an embodiment of a piezoelectric transformer of the present invention. As shown in FIG. 1(a), the piezoelectric transformer 107 connects an input piezoelectric element 103, which is a first piezoelectric element, and an output piezoelectric element 106, which is a second piezoelectric element, to a first member 109 and a second piezoelectric element. It has a configuration in which it is sandwiched between members 110. The first piezoelectric element and the second piezoelectric element are composed of polarized piezoelectric ceramics and a plurality of electrodes, and the piezoelectric ceramics and electrodes are laminated in the same direction as the members, and the polarization direction of the piezoelectric ceramics is the laminated direction of the members. Assume that it is the same as A simple electrode for polarization treatment may be bonded to the piezoelectric ceramic. Further, as shown in FIG. 1(b), two first piezoelectric elements 103 and two second piezoelectric elements 106 are arranged with the same polarity facing each other, and the first member 109 and the second member 110. An example of the first member 109 and the second member 110 is a metal cylinder, but the material and shape are not particularly limited as long as the first piezoelectric element 103 and the second piezoelectric element 106 can be sandwiched therebetween. When electrical energy is applied as an alternating voltage to the input terminal 102 of the input section 101, the electrical energy is converted into elastic energy by the inverse piezoelectric effect of the input piezoelectric element 103. This elastic energy becomes electric energy again due to the positive piezoelectric effect of the output piezoelectric element 106 of the output section 104 and is taken out from the output terminal 105. That is, the configuration is such that a voltage is applied to the input section 101 and a voltage generated at the output section 104 is extracted through mechanical vibration.

Furthermore, the piezoelectric element according to the present invention is composed of an electrode and piezoelectric ceramics. The electrode and the piezoelectric ceramic may be bonded together or may be stacked one on top of the other.

The shape of the piezoelectric transformer 107 in the present invention may be, for example, a cylindrical shape having an outer diameter R, or a shape having a hole in the center of a cylindrical column having an outer diameter R to pass a bolt having an inner diameter r. By tightening nuts from both ends of the structure and applying pressure (pressurization), the resistance to destruction of the piezoelectric element against the tensile stress of the piezoelectric transformer is improved, and by increasing the input power, it is possible to use it at high power. . FIG. 2 shows the configuration of the pressurizing mechanism in the piezoelectric transformer of the present invention. FIG. 2(a) is an example of a pressurizing mechanism in which nuts are tightened from both ends of the first member and the second member, and FIG. 2(b) is a sectional view of FIG. 2(a). In order to pass the shaft 118 inside the piezoelectric transformer 107, a hole larger than the diameter of the shaft 118 is provided in the first member 109, the first piezoelectric element 103, the second piezoelectric element 106, and the second member 110. There is a need.

Further, when the shaft 118 and the first piezoelectric element 103 including the piezoelectric ceramic 114 and the electrode 116 and the second piezoelectric element 106 having a similar structure come into contact with each other, the vibrations of the first piezoelectric element 103 and the second piezoelectric element 106 occur. It becomes a factor that inhibits Therefore, a space is always required between the shaft 118 and the first piezoelectric element 103 and the second piezoelectric element 106. Further, when a metal shaft 118 is used, there is a risk that a short circuit may occur if it comes into contact with the first piezoelectric element 103 and the second piezoelectric element 106. The shaft 118 is provided with threaded grooves at both ends of the piezoelectric transformer, and by tightening with nuts, it becomes possible to apply pressure to the piezoelectric transformer. In this way, as the pressurizing means, as schematically shown in FIG. 2(c), a pressurizing mechanism 112 (shaded area) is used, which is tightened with bolts, nuts, etc. using a through hole prepared at the center of the piezoelectric transformer 107. A method of providing is preferred. At this time, it is important to maintain the in-plane uniformity of the stress applied to the piezoelectric element as high as possible during the stretching vibration used when driving the element, to ensure high output efficiency, and to improve durability. From this point of view, it is more preferable that the cross-sectional shape of the bolt head and nut of the pressurizing mechanism 112 used for tightening be equal to the cross-sectional shape of the piezoelectric transformer 107 (FIG. 2(d)). Furthermore, a structure in which the bolt head and nut are embedded in the pressurizing mechanism 112 (FIG. 2(e)) is more preferable because the piezoelectric transformer 107 can be made smaller.

The resonance of expansion and contraction is used to drive the piezoelectric transformer. FIG. 3 shows a schematic diagram of the displacement amount and stress distribution during first- to fourth-order stretch resonance in a configuration in which both ends of the piezoelectric transformer are free ends. By installing the input section and the output section at positions where the stress is maximum during stretch resonance used when driving the element, it is possible to obtain a piezoelectric transformer with high conversion efficiency of output power to input power. Although we have described a configuration in which both ends are free ends, even in a structure in which one end is a fixed end or both ends are fixed ends, it is possible to install the input part and the output part at the position where the stress is maximum at the time of expansion/contraction resonance. , it is possible to obtain a piezoelectric transformer with high conversion efficiency of output power to input power.

For estimating the position where the stress is maximum at the time of the expansion/contraction resonance of the piezoelectric transformer, the finite element method calculation can be performed, for example. An example of a finite element method package software is "ANSYS" (ANSYS Inc.).

The piezoelectric transformer according to the present invention is shown in FIG. 2 as an example. That is, a laminate in which a first member, a first piezoelectric element, a second piezoelectric element, and a second member are laminated in this order, and the first member and second member of the laminate are stacked in the stacking direction. The piezoelectric transformer includes at least a pressurizing mechanism that tightens the piezoelectric transformer and the pressurizing mechanism. Furthermore, the ratio (k ₃₃ /k ₃₁ ) of the electromechanical coupling coefficient k ₃₃ to the electromechanical coupling coefficient k ₃₁ of the first piezoelectric element and the second piezoelectric element is 2.0 or more. . When (k ₃₃ /k ₃₁ ) is 2.0 or more, friction loss at the interface between the piezoelectric ceramic and peripheral members such as electrode plates and metal cylinders is reduced, and heat generation can be suppressed. In other words, it is possible to obtain a piezoelectric transformer that efficiently converts vibration energy and has high conversion efficiency of output power to input power. The electromechanical coupling coefficient k is one of the quantities representing the magnitude of the piezoelectric effect, and represents the efficiency of converting electrical energy into mechanical energy. The larger the value of k, the greater the effect, and can be calculated according to the Japan Electronics and Information Technology Industries Association (JEITA) standard EM-4501A. In addition, as shown in FIG. 4(a), the electromechanical coupling coefficient _k31 represents the effect of long-side elongation vibration, which is a vibration mode in one axis direction with respect to the three-axis polarization direction, and is used in the present invention. In a piezoelectric transformer, it represents the effect of vibration in the direction perpendicular to the stacking direction of the members. On the other hand, as shown in FIG. 4(b), the electromechanical coupling coefficient _k33 represents the effect of longitudinal vibration, which is a vibration mode in the three axial directions with respect to the polarization direction in the three axial directions, and in the piezoelectric transformer of the present invention. represents the effect of vibration in the stacking direction of the members.

Further, the piezoelectric transformer according to the present invention is shown in FIG. 2 as an example. That is, a laminate in which a first member, a first piezoelectric element, a second piezoelectric element, and a second member are laminated in this order, and the first member and second member of the laminate are stacked in the stacking direction. The piezoelectric transformer includes at least a pressurizing mechanism that tightens the piezoelectric transformer and the pressurizing mechanism. The electromechanical coupling coefficient k ₃₁ of the first piezoelectric element and the second piezoelectric element is less than 30% and k ₃₃ is 50% or more. When the electromechanical coupling coefficient k ₃₁ is less than 30%, friction loss at the interface between the piezoelectric ceramic and peripheral members such as electrode plates and metal cylinders is reduced, and heat generation can be suppressed. Further, when _k33 is 50% or more, it is possible to increase the vibration energy generated during driving in the input section and the output section, and it is possible to increase the output power. In this way, by having an electromechanical coupling coefficient _k31 of less than 30% and _k33 of 50% or more, a piezoelectric transformer that converts vibration energy efficiently and has high output and high conversion efficiency of output power with respect to input power can be obtained. be able to. Further, the piezoelectric transformer according to the present invention is shown in FIG. 2 as an example. That is, a laminate in which a first member, a first piezoelectric element, a second piezoelectric element, and a second member are laminated in this order, and the first member and second member of the laminate are stacked in the stacking direction. The piezoelectric transformer includes at least a pressurizing mechanism that tightens the piezoelectric transformer and the pressurizing mechanism. Further, the ratio (k ₃₃ /k ₃₁ ) of the electromechanical coupling coefficient k ₃₃ to the electromechanical coupling coefficient k ₃₁ of the first piezoelectric element and the second piezoelectric element is 2.0 or more. Further, k ₃₃ /k ₃₁ satisfies the above-mentioned ratio, and the electromechanical coupling coefficient k ₃₁ of the first piezoelectric element and the second piezoelectric element is less than 30% and k ₃₃ is 50% or more. It is characterized by That is, (k ₃₃ /k ₃₁ ) is 2.0 or more, the electromechanical coupling coefficient k ₃₁ of the first piezoelectric element and the second piezoelectric element is less than 30%, and k ₃₃ is 50% or more. This reduces friction loss at the interface between the piezoelectric ceramic and peripheral members such as electrode plates and metal cylinders, making it possible to suppress heat generation. Furthermore, the vibration energy generated during driving in the input section and the output section can be increased. Therefore, it becomes possible to increase the output power. The piezoelectric transformer can convert vibration energy efficiently and have high efficiency in converting output power to input power.

In this way, there is no known piezoelectric transformer using a piezoelectric element that takes into account the ratio ( _{k 33 /} k ₃₁ ) of the electromechanical coupling coefficient k ₃₃ to the electromechanical coupling coefficient k 31 of the piezoelectric element.

The ratio (k ₃₃ /k ₃₁ ) of the electromechanical coupling coefficient k ₃₃ to the electromechanical coupling coefficient k ₃₁ of the first piezoelectric element and the second piezoelectric element is more preferably 2.2 or more, and even more preferably 2.2. It is 3 or more.

In the piezoelectric transformer according to the present invention, it is preferable that a third member is laminated between the first piezoelectric element and the second piezoelectric element. By laminating the third member between the first piezoelectric element and the second piezoelectric element, it becomes possible to utilize, for example, the secondary expansion/contraction mode shown in FIG. 3(b). In the secondary expansion/contraction mode, each piezoelectric element on the input side and the piezoelectric element on the output side can be installed at a position where the stress is maximum, so a piezoelectric transformer with high power conversion efficiency can be obtained.

(Pressurization mechanism)
In the piezoelectric transformer according to the present invention, it is preferable that the pressurizing mechanism penetrates the laminate. As shown in FIG. 2, a structure may be adopted in which a pressurizing mechanism penetrates the laminate and applies pressure from both ends of the structure. With such a configuration, the components of the piezoelectric transformer, including the pressurization mechanism, become an integrated resonator, which reduces vibration loss, resulting in a piezoelectric transformer with high power conversion efficiency. Can be done.

In the piezoelectric transformer according to the present invention, it is preferable that the pressurizing mechanism is embedded in the laminate. By embedding the pressurizing mechanism in the laminate as shown in FIG. 2(c), it is possible to maintain the in-plane uniformity of the stress applied to the piezoelectric element as high as possible during the stretching vibration used when driving the element. As a result, the piezoelectric transformer can be improved in durability and has a high conversion efficiency of output power to input power, which is preferable. Further, it is preferable that the piezoelectric transformer is embedded in the pressurizing mechanism laminate because it allows the piezoelectric transformer to be miniaturized.

In the piezoelectric transformer according to the present invention, it is preferable that the piezoelectric element is formed by laminating an even number of piezoelectric ceramic plates and a plurality of electrodes. In Fig. 1(a), the piezoelectric elements of the input section and the output section are each made of one piezoelectric ceramic, whereas in Fig. 1(b), the input section and the output section are each made of two piezoelectric ceramics. ing. When the same voltage is supplied from the drive circuit, vibration energy is generated by one piezoelectric ceramic in FIG. 1(a), whereas vibration energy is generated by two piezoelectric ceramics in FIG. 1(b). Therefore, by forming the piezoelectric element into a structure in which an even number of plate-shaped piezoelectric ceramics and a plurality of electrodes are laminated, a high-output piezoelectric transformer can be obtained.

In the piezoelectric transformer according to the present invention, it is preferable that the third member is an insulator. Since the third member is an insulator, the input side circuit and the output side circuit are insulated from each other. Such an insulating structure prevents electricity from flowing directly on the input side and protects the circuit connected to the output side. Further, it is possible to prevent an unexpected backflow of electricity from the output side from being transmitted to the input side. For this reason, it is more preferable that the third member is an insulator.

(Piezoelectric element)
The Young's modulus of the piezoelectric element according to the present invention at room temperature is preferably 100 GPa or more and 200 GPa or less. Since there is a positive correlation between Young's modulus and resonant frequency, the larger the Young's modulus, the higher the resonant frequency. When producing an element with a constant resonant frequency, a piezoelectric element with a large Young's modulus can be made smaller than a piezoelectric element with a small Young's modulus. Young's modulus can be calculated from the sound velocity and density of the vibration form, and the density can be measured, for example, by the Archimedes method.

It is preferable that the Pb component contained in the piezoelectric element according to the present invention is less than 1000 ppm. For example, it has been pointed out that when a piezoelectric element is disposed of and exposed to acid rain or left in a harsh environment, the lead component in the piezoelectric ceramic contained in the piezoelectric element may leach into the soil and harm the ecosystem. ing. Considering the influence of the lead component on the environment, the piezoelectric element of the present invention is preferably of a non-lead type, and if the lead component contained in the piezoelectric element is less than 1000 ppm, the influence of lead component elution can be reduced. , it can be said to be a lead-free piezoelectric element. The lead content can be evaluated based on the lead content relative to the total weight of the piezoelectric ceramic determined by, for example, X-ray fluorescence analysis (XRF) or ICP emission spectrometry.

(piezoelectric ceramics)
In this specification, "ceramics" refers to an aggregate (also referred to as a bulk body) of crystal grains whose basic component is a metal oxide and is baked and solidified by heat treatment. Also includes those processed after sintering. However, powder and slurry in which powder is dispersed are not included in this term.

The piezoelectric ceramic according to the present invention is preferably composed of a perovskite metal oxide. Perovskite metal oxides have excellent piezoelectric properties, so they can be used as high-output piezoelectric transformers.

Perovskite metal oxides are generally expressed by the chemical formula _ABO3 . In the perovskite metal oxide, elements A and B occupy specific positions in the unit cell called A site and B site in the form of ions, respectively. For example, in a cubic unit cell, element A is located at the apex of the cube, and element B is located at the center of the body. The O element occupies the face-centered position of the cube as an anion of oxygen. By distorting the unit cell in the [001], [011], or [111] direction of the cubic unit cell, a crystal lattice of a tetragonal, orthorhombic, or rhombohedral perovskite structure is formed, respectively.

In the present invention, the molar ratio (A/B) of A (mol) representing the A site and B (mol) representing the B site expressed by the chemical formula of ABO ₃ is expressed using a. Even if a is a value other than 1, it is within the scope of the present invention as long as the metal oxide has a perovskite structure as its main phase.

In addition, in the chemical formula of ABO ₃ , the molar ratio of the B site element to the O element is 1:3, but even if the ratio of the element amounts deviates slightly, for example within 1%, the metal oxide may form a perovskite structure. If it is the main phase, it falls within the scope of the present invention.

Whether the metal oxide has a perovskite structure can be determined from, for example, X-ray diffraction or electron diffraction of piezoelectric ceramics. As long as the perovskite structure is the main crystalline phase, the piezoelectric ceramic may contain other crystalline phases as a subsidiary.

The types of compounds constituting the piezoelectric ceramics according to the present invention are not particularly limited, and examples thereof include lead zirconate titanate (PZT), barium titanate, barium calcium titanate, sodium bismuth titanate, lead titanate, lithium niobate, Potassium sodium niobate, bismuth ferrate, and metal oxides containing these as main components can be used. Considering the influence of the lead component on the environment, it is more desirable that the piezoelectric ceramic of the present invention be a lead-free metal oxide. When the lead component contained in the piezoelectric ceramic is less than 1000 ppm, the influence of lead component elution can be reduced, so it can be said to be a lead-free piezoelectric ceramic.

(Composition of piezoelectric ceramics)
The piezoelectric ceramic according to the present invention preferably contains Ba and Ti. For example, barium titanate is an example of a perovskite metal oxide containing Ba and Ti. Piezoelectric ceramics containing barium titanate as a main component have a large absolute value of piezoelectric constant d. Therefore, since the voltage required to obtain the same amount of distortion can be reduced, a piezoelectric transformer with high power conversion efficiency can be obtained.

The piezoelectric constant of a piezoelectric ceramic can be determined by calculation based on the Japan Electronics and Information Technology Industries Association (JEITA) standard EM-4501A from the measurement results of the density, resonance frequency, and anti-resonance frequency of the ceramic. The resonant frequency and anti-resonant frequency can be measured using an impedance analyzer, for example, after providing a pair of electrodes on the ceramic.

It is preferable that the piezoelectric ceramic according to the present invention further contains Ca and Zr. By containing Ca, the phase transition temperature from tetragonal to orthorhombic (hereinafter referred to as Tto) when the temperature decreases and the phase transition temperature from orthorhombic to tetragonal when the temperature increases (hereinafter referred to as Tot) are lowered. . Moreover, by containing Zr, the temperature of Tto and Tot increases. Furthermore, the Curie temperature (hereinafter referred to as Tc), which is the phase transition temperature from tetragonal to cubic, decreases and the dielectric constant increases, so that the absolute value of the piezoelectric constant d increases. In this way, by including Ca and Zr, the influence of the phase transition temperature in the operating temperature range is reduced, thereby improving the temperature stability of piezoelectric characteristics, and making it possible to provide a piezoelectric transformer with high power conversion efficiency.

The Curie temperature Tc is generally the temperature above which the piezoelectricity of a piezoelectric material disappears, and in this specification, it refers to the ferroelectric phase (tetragonal phase) and the paraelectric phase (cubic phase). The temperature at which the dielectric constant becomes maximum near the transition temperature is defined as Tc. The Tc of a piezoelectric material made of a metal oxide whose main component is barium titanate is approximately 100°C to 130°C.

The piezoelectric ceramic according to the present invention is more preferably a piezoelectric ceramic having the following composition ratio. That is, x, the molar ratio of Ca to the sum of Ba and Ca, satisfies 0.02≦x≦0.30, and y, the molar ratio of Zr to the sum of Ti and Zr, satisfies 0.020≦y≦0. 095, and y≦x. It is more preferable for the value of x to be 0.02 or more because the temperature dependence of the piezoelectric constant becomes smaller. On the other hand, when x is smaller than 0.30, solid solution of Ca is promoted and the firing temperature can be lowered. Furthermore, it is more preferable that the value of y, which is the molar ratio of Zr to the sum of the contents of Ti and Zr, is 0.020≦y≦0.095. By setting the value of y to 0.02 or more, the piezoelectric constant in the operating temperature range (for example, −30° C. to 60° C.) becomes large. On the other hand, by setting y to 0.095 or less, T _C becomes high, for example, 100°C or more, and depolarization when used at high temperatures is further suppressed, so the guaranteed operation temperature range of the piezoelectric device becomes more As the width increases, the deterioration of the piezoelectric constant over time becomes smaller.

In this way, by setting 0.02≦x≦0.30 and 0.020≦y≦0.095, and y≦x, the temperature dependence of the piezoelectric constant is small, and the piezoelectric constant is large and the electric power is A piezoelectric transformer with high conversion efficiency can be used.

The piezoelectric ceramic according to the present invention is a piezoelectric material further containing Mn, and the content of the Mn based on 100 parts by weight of the oxide is 0.02 parts by weight or more and 0.40 parts by weight or less in terms of metal. is preferred.

It is more preferable for the Mn content to be in the range of 0.02 parts by weight or more and 0.40 parts by weight or less in terms of metal because the value of Qm at room temperature can be increased. Generally, Qm is a coefficient representing elastic loss due to vibration when evaluating a piezoelectric material as a vibrator, and the magnitude of Qm is observed as the sharpness of a resonance curve in impedance measurement. In other words, Qm is a constant representing the sharpness of resonance of the vibrator. When Qm is high, less energy is lost due to vibration. By improving Qm, when a piezoelectric material is used as a piezoelectric element and a voltage is applied to drive the piezoelectric element, the loss is small and the piezoelectric element can be made efficient. Qm can be measured according to the Japan Electronics and Information Technology Industries Association (JEITA) standard EM-4501A.

Mn has a property of varying valence between divalence and tetravalence, and plays a role in compensating for defects in charge balance in piezoelectric ceramics. By setting the Mn content to 0.02 parts by weight or more, the concentration of oxygen vacancies in the crystal lattice of the piezoelectric ceramic increases, and the residual stress generated between crystal grains due to domain switching of non-180 degree domains is reduced. Therefore, it is considered that the value of Qm increases further. On the other hand, by setting the Mn content to 0.40 parts by weight or less, solid solution of Mn can be promoted and insulation resistance can be further increased. It is preferable that the insulation property is improved because long-term reliability of the piezoelectric element can be ensured when the piezoelectric material is used as a piezoelectric element and a voltage is applied to drive the piezoelectric element.

Mn is not limited to metal Mn, but may be included in the piezoelectric ceramic as a Mn component, and the form of its inclusion does not matter. For example, it may be in solid solution at the B site, or it may be included in the grain boundaries. Alternatively, the Mn component may be contained in the piezoelectric ceramic in the form of a metal, ion, oxide, metal salt, complex, or the like. More preferably, Mn is present from the viewpoint of insulation and ease of sintering. Generally, the valence of Mn can be 4+, 2+, or 3+, but when the valence of Mn is lower than 4+, Mn becomes an acceptor. When Mn is present as an acceptor in a perovskite structure crystal, oxygen vacancies are formed in the crystal. When oxygen vacancies form defect dipoles, the Qm of piezoelectric ceramics can be improved. In order for Mn to exist with a valence lower than 4+, it is preferable that a trivalent element exists at the A site. A preferred trivalent element is Bi. On the other hand, the valence of Mn can be evaluated by measuring the temperature dependence of magnetic susceptibility.

It is preferable that the piezoelectric ceramic according to the present invention contains Na and Nb. An example of piezoelectric ceramics containing Na and Nb is a solid solution of sodium niobate (NaNbO3) and barium titanate (BaTiO3) (hereinafter referred to as "NN-BT"). Since NN-BT does not substantially contain potassium, which causes difficulty in sintering and low moisture resistance, it has the advantage that its piezoelectric properties hardly change over time. Furthermore, when NN-BT is used in a piezoelectric transformer, there is no point (temperature) that causes a phase transition in the crystal structure within the operating temperature range of the piezoelectric transformer (for example, from 0°C to 80°C), so performance may vary depending on the operating temperature. Another advantage is that there is almost no significant fluctuation.

(Electronics)
The piezoelectric transformer device of the present invention can be used by providing an exterior part 113 as shown in FIGS. 9(a) and 9(b), incorporating it into an electronic device, and connecting it to an input drive circuit and an output circuit (external load). is also preferable. Because it is capable of high output and high conversion efficiency, it can be made smaller than conventional piezoelectric transformers, making it possible to miniaturize electronic devices.

(Example)
The piezoelectric transformer of the present invention will be specifically described below with reference to Examples, but the present invention is not limited to the following Examples.

(piezoelectric ceramics)
Two types of PZT (lead zirconate titanate), two types of NN-BT (solid solution of sodium niobate and a small amount of barium titanate) and one type of BCTZ-Mn (Ca, Zr and Mn) polarized as piezoelectric ceramics. (barium titanate) was prepared. The two types of PZT were designated as PZT_1 and PZT_2, and the two types of NN-BT were designated as NN-BT_1 and NN-BT_2. The density was then measured using the Archimedes method. In addition, when impurity elements contained in two types of piezoelectric ceramics, NN-BT and BCTZ-Mn, were measured by semi-quantitative analysis using GDMS (glow discharge mass spectrometry), it was found that Pb contained in the piezoelectric ceramics was less than 50 ppm. Ta.

The electromechanical coupling coefficient k ₃₁ , the electromechanical coupling coefficient k ₃₃ , the ratio of the electromechanical coupling coefficient k ₃₃ (k ₃₃ /k ₃₁ ), the frequency constant N ₃₁ and the Young's modulus Y ₁₁ were evaluated as the basic piezoelectric properties of piezoelectric ceramics. did. In order to evaluate the piezoelectric basic characteristics, gold (Au) electrodes with a thickness of 400 nm were formed on both the front and back surfaces of the piezoelectric ceramic by DC sputtering. Note that a 30 nm thick titanium (Ti) film was formed as an adhesive layer between the electrode and the piezoelectric ceramic. After that, it was processed into a prismatic shape of length 0.9 mm x width 0.9 mm x thickness 4.9 mm based on the Japan Electronics and Information Technology Industries Association (JEITA) standard EM-4501A, and the electromechanical coupling coefficient k ₃₃ was measured. I did it. Further, based on the same standard, it was processed into a rectangular plate shape of 10 mm in length x 2.5 mm in width x 0.5 mm in thickness, and the electromechanical coupling coefficient _k31 and resonance frequency were measured using an impedance analyzer. A frequency constant N ₃₁ and a Young's modulus Y ₁₁ were calculated from the density measurement results and the resonance frequency measurement results. Table 1 shows the basic piezoelectric properties of these piezoelectric ceramics.

(Fabrication of piezoelectric transformer)
(Structure 1)
As structure 1, as shown in FIG. 5(a), a piezoelectric transformer 107 was used in which an input section 101 and an output section 104 were stacked. The input section 101 includes a first member 109, a first piezoelectric element (input piezoelectric element) 103, and an input terminal 102, and the input terminal 102 is connected to an electrode constituting the first piezoelectric element 103. Further, the output section 104 includes a second member 110, a second piezoelectric element (output piezoelectric element) 106, and an output terminal 105, and the output terminal 105 is connected to an electrode constituting the second piezoelectric element 106. SUS was used for the first member and the second member. The first piezoelectric element 103 and the second piezoelectric element are constructed by laminating two piezoelectric ceramics to which simple electrodes for polarization processing are bonded, and three 0.2 mm thick copper plates (not shown) as electrodes. And so. In addition, the electrodes between the input section and the output section were common, and a total of five copper plates were used. The first member constituting the piezoelectric transformer 107 has a diameter of 13 mm and a length of 29.7 mm, and the piezoelectric ceramics constituting the first piezoelectric element 103 and the second piezoelectric element 106 have a diameter of 13 mm and a length of 2.5 mm. The second member 110 had a diameter of 13 mm, a length of 24.3 mm, and a total length of 65 mm. Further, in order to provide a pressurizing mechanism that applies pressure to the structure of the piezoelectric transformer 107 from both ends, a through hole with an inner diameter of 6 mm was provided in each component constituting the piezoelectric transformer 107. Then, a SUS shaft was passed through the through hole, both ends were fixed with nuts, and the piezoelectric transformer was tightened with a torque of 10 N·m.

FIG. 6A schematically shows the relationship between the displacement amount during the primary stretching vibration and the input section 101 and the output section 104 in this configuration. The piezoelectric transformer is configured with both ends as free ends, and the center position of the first piezoelectric element 103 constituting the input section 101 is such that the absolute value of the displacement amount of the first-order stretching vibration becomes 0, that is, the stress becomes the largest. position.

(Structure 2)
As structure 2, as shown in FIG. 5(b), a piezoelectric transformer 107 was used in which an input section 101, a third member 111, and an output section 104 were stacked. The input section 101 includes a first member 109, a first piezoelectric element (input piezoelectric element) 103, and an input terminal 102, and the input terminal 102 is connected to an electrode constituting the first piezoelectric element 103. SUS was used for the first member, the second member, and the third member. Further, the output section 104 includes a second member 110, a second piezoelectric element (output piezoelectric element) 106, and an output terminal 105, and the output terminal 105 is connected to an electrode constituting the second piezoelectric element 106. The first piezoelectric element 103 and the second piezoelectric element had a structure in which two piezoelectric ceramics and three copper plates (not shown) with a thickness of 0.2 mm were laminated as electrodes, and a total of six copper plates were used. The first member constituting the piezoelectric transformer 107 has a diameter of 15 mm and a length of 3.5 mm, and the piezoelectric ceramics constituting the first piezoelectric element 103 and the second piezoelectric element 106 have a diameter of 15 mm and a length of 1.5 mm. The second member 110 had a diameter of 15 mm and a length of 3.5 mm, and the third member had a diameter of 15 mm and a length of 7 mm, making the total length 21 mm. Further, in order to provide a pressurizing mechanism that applies pressure to the structure of the piezoelectric transformer 107 from both ends, a through hole with an inner diameter of 6 mm was provided in each component constituting the piezoelectric transformer 107.

FIG. 6(b) schematically shows the relationship between the displacement amount during the second-order stretching vibration and the input section 101 and the output section 104 in this configuration. The piezoelectric transformer has a configuration in which both ends are free ends, and the center position of the first piezoelectric element 103 that constitutes the input section 101 and the center position of the second piezoelectric element 106 that constitutes the output section 104 correspond to the displacement of the second-order stretching vibration. The absolute value of the quantity was set to 0, that is, the position where the stress was the largest.

Using the piezoelectric ceramics shown in Table 1, piezoelectric transformers of Examples 1 to 5 and Comparative Example 1 were manufactured as shown in Table 2.

(Characteristics evaluation of piezoelectric transformer)
To evaluate the characteristics of the piezoelectric transformer, an AC voltage was applied to the input terminal 102 shown in FIG. 5, and the power output from the output terminal 105 was measured. The input power is calculated from the voltage value and current value at the input terminal, the output power is calculated from the voltage value at the output terminal and the load resistance (not shown) connected to the output terminal, and the output power of the piezoelectric transformer is calculated from the ratio of the output power to the input power. Conversion efficiency was calculated.

The characteristics evaluation results of the piezoelectric transformers in Comparative Example 1 and Examples 1 to 4 are shown in FIGS. 7 and 8. FIG. 7 shows the conversion efficiency of output power to input power at each output power of the piezoelectric transformer. Also, Figure 8(a) shows the conversion efficiency of output power to input power when the output power of the piezoelectric transformer is 10W, and Figures 8(b) and 8(c) show the output power to input power when the output power of the piezoelectric transformer is 30W. It represents the power conversion efficiency.

As shown in the results of FIGS. 7 and 8(a), the ratio of electromechanical coupling coefficient k ₃₃ of piezoelectric ceramics (k ₃₃ /k ₃₁ ) was 1.8 in Comparative Example 1, whereas Example 1 4 is more advantageous in the following points. That is, Examples 1 to 4 in which the ratio of the electromechanical coupling coefficient k ₃₃ (k ₃₃ /k ₃₁ ) of the piezoelectric ceramic is 2.0 or more have higher conversion efficiency of output power to input power of the piezoelectric transformer.

Comparing Example 1 with Example 2 to Example 4, in Examples 2 to 4, the electromechanical coupling coefficient k ₃₁ of the piezoelectric ceramics satisfies less than 30% and k ₃₃ satisfies 50% or more. Therefore, as shown in FIGS. 8(b) and 8(c), the conversion efficiency of output power to input power is higher than in Example 1 even at high output power of 30 W or more.

Comparing Examples 2 to 4, as shown in FIG. 8(a), the conversion efficiency of output power to input power increases in descending order of the ratio of electromechanical coupling coefficient k ₃₃ (k ₃₃ /k ₃₁ ) of piezoelectric ceramics. is higher in Example 4 than in Example 3, and higher in Example 2 than in Example 4.

Further, the power conversion efficiency of Example 5, which is Structure 2, was 89% when the output power was 10W, 85% when the output power was 30W, and 82% when the output power was 50W. Comparing Example 2 and Example 5, Example 5 has a third member between the input section and the output section. In Example 2, the center of the first piezoelectric element (input piezoelectric element) was set as the position where the stress was greatest. On the other hand, in Example 5, the center of each of the first piezoelectric element (input piezoelectric element) and the second piezoelectric element (output piezoelectric element) is set at the position where the stress is greatest, thereby achieving high conversion efficiency. Indicated.

Next, in the configuration of the piezoelectric transformer of Example 5, a piezoelectric transformer having a shape in which a concave portion was provided and a nut was embedded in the first member and the second member made of SUS, as shown in FIG. 2(e), was manufactured. In addition, the input and output sections were installed at positions where the stress would be maximum during stretch resonance. When the characteristics of the piezoelectric transformer were evaluated as in Examples 1 to 5, conversion efficiency of output power to input power equivalent to that in Example 5 was confirmed. By adopting a structure in which the pressurizing mechanism is embedded in the first member and the second member, the piezoelectric transformer can be downsized.

In this Structure 2, SUS was used as the third member, but it is also possible to use not only metal but also an insulator. Since the third member is an insulator, the input side circuit and the output side circuit are insulated from each other. Such an insulating structure prevents electricity from flowing directly on the input side and protects the circuit connected to the output side. Further, it is possible to prevent an unexpected reverse flow of electricity from the output side from being transmitted to the input side.

By using the piezoelectric transformer of the present invention, heat generation at the interface between the piezoelectric ceramic and peripheral members such as electrode plates and metal cylinders can be suppressed, and high power conversion efficiency can be exhibited. The piezoelectric transformer of the present invention can also be used in electronic equipment using piezoelectric transformers.

101 Input part 102 Input terminal 103 Input piezoelectric element 104 Output part 105 Output terminal 106 Output piezoelectric element 107 Piezoelectric transformer 109 First member 110 Second member 111 Third member 112 Pressurizing mechanism 113 Exterior part 114 Piezoelectric ceramics (For the first piezoelectric element)
115 Piezoelectric ceramics (for second piezoelectric element)
116 Electrode 117 Nut 118 Shaft

Claims (9)

A laminate in which a first member, a first piezoelectric element having an electrode and a piezoelectric ceramic , a second piezoelectric element having an electrode and a piezoelectric ceramic , and a second member are laminated in this order; A piezoelectric transformer comprising at least a pressurizing mechanism that tightens the first member and the second member together in the stacking direction,
The piezoelectric ceramic is composed of a perovskite metal oxide containing Ba, Ca, Ti, and Zr, or containing Na and Nb,
The electromechanical coupling coefficient _k31 of the first piezoelectric element and the second piezoelectric element is 23% or more and 26 % or less , and _k33 is 53 % or more and 58% or less ,
The ratio (k ₃₃ /k ₃₁ ) of the electromechanical coupling coefficient k ₃₃ to the electromechanical coupling coefficient k ₃₁ of the first piezoelectric element and the second piezoelectric element is 2.1 or more and 2.3 or less ,
Young's modulus Y ₁₁ at room temperature of the first piezoelectric element and the second piezoelectric element is 116 GPa or more and 123 GPa or less,
The frequency constant N ₃₁ of the first piezoelectric element and the second piezoelectric element is 2210 Hz·m or more and 2610 Hz·m or less,
A piezoelectric transformer, wherein a Pb component contained in the first piezoelectric element and the second piezoelectric element is less than 1000 ppm. The piezoelectric ceramic is a piezoelectric ceramic containing Ba, Ca, Ti, and Zr,
x, which is the molar ratio of Ca to the sum of Ba and Ca, is 0.02≦x≦0.30, and y, which is the molar ratio of Zr to the sum of Ti and Zr, is 0.020≦ y≦0.095,
The piezoelectric transformer according to claim 1 , wherein y≦x. The piezoelectric ceramics is
Furthermore, a piezoelectric material containing Mn,
2. The piezoelectric transformer according to claim 1 , wherein the content of the Mn based on 100 parts by weight of the oxide is 0.02 parts by weight or more and 0.40 parts by weight or less in terms of metal. The piezoelectric transformer according to claim 1 or 2, wherein a third member is laminated between the first piezoelectric element and the second piezoelectric element. The piezoelectric transformer according to claim 1 or 2, wherein the pressurizing mechanism penetrates the laminate. The piezoelectric transformer according to claim 1 or 2, wherein the pressurizing mechanism is embedded in the laminate. 3. The piezoelectric transformer according to claim 1, wherein each of the first piezoelectric element and the second piezoelectric element is formed by laminating an even number of plate-shaped piezoelectric ceramics and a plurality of electrodes. The piezoelectric transformer according to claim 4 , wherein the third member is an insulator. An electronic device comprising the piezoelectric transformer according to claim 1 or 2, and a drive circuit that supplies an alternating voltage to the piezoelectric transformer.

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